Origin and distribution of mineral species in limestone caves

EARTH SCIENCE JOURNAL
Vol. 5, No. 1, 1971
ORIGIN AND DISTRIBUTION O'F MINERAL SPECIES IN
LIMESTONE CAVES
PAUL L. BROUGHTON
Department of Geology*
Virginia Polytechnic Institute and State University,
Blacksburg, Virginia
Abstract
More than 80 mineral species are known to result from secondary chemical reactions
in solutional limestone caverns. These minerals are transitional and end-products of carbonate
wall rock interactions with ground water and organic residue. Bacteria are often critical
in fixing various ions in these reactions. Ultimate source of metallic ions is usually the sulphide
minerals weathering in overlying strata, and brought to the cave environment by circulating
ground waters. Hydrothermal solutions may bring heavy metallic ion concentrations from
deeper strata. About one-third of the known cave mineral species occur in stalactitic form.
* Present address: Dept. of Mineral Resources, Industrial Minerals Division, Government
Administration Building, Regina, Saskatchewan, Canada.
INTRODUCTION
Since the Second World War a vigorous scientific interest has focused on the
chemical and physical processes active in the cavern environment. The few caves
that have been systematically studied were found to contain enti:rely unsuspected
minerals. Further studies on determining the number and distribution of cave
minerals will increase our understanding of the bulk composition of mineralising
solutions. When one considers the interactions between the carbonate minerals in
the wall rock with organic residue or with mineralised ground water, more than
80 mineral species are known. In this paper only minerals that are formed within
solutional limestone-dolomite caves are considered. These are the result of low
temperature and low pressure mineralisation reactions after the formation of the
cavernous void itself, thus constituting a secondary process. Detrital mineral grains
washed in from outside the cave are not considered in this respect.
The follo1wing tabulation of known cave minerals, their distribution and mode
of formation, has been divided into categories based on the chemical composition
class.
THE CARBONATE MINERALS
There are three polymorphs of calcium carbonate which occur in nature:
calcite, aragonite and vaterite in order of decreasing stability. Vaterite is very
rare, and unknown in the cavern environment.
The occurrence of aragonite in caves is probably the
investigated cave phenomena. Chemical data predict that
stable at a low cave temperature (say 25°C.) when the
atmospheres. Aragonite, however, is amongst the most
with world-wide distribution.
most puzzling and most
aragonite could only be
pressure is above 3000
common cave minerals
Consequently, the mechanism for its formation is believed to be controlled
by trace elements, the carbonate-ion itself, supersaturation, and fundamental
36
mechanisms of nucleation and crystal growth of the pure polymorphs of calcium
carbonate, in the cave envi1romnent. Aragonite is about 16 per cent more soJuble
than calcite, and hence unstable with respect to it. However, both are stable with
respect to the ions in solution during precipitations when the supersaturation with
respect to calcite exceeds the solubility of aragonite. Curl (1962) proposed that
under these conditions, the relative rates of crystal nucleation and crystal growth
determine which polymorph will dominate.
It has been found that certain trace am0'unts of impurities and certain conditions
of temperature can have a pr0'found effect 0'n whether aragonite or calcite appears.
Murray's (1954) experimental studies showed increases in the percentage of
aragonite with the mcrease of magnesium concentrati0'n. Wray and Daniels (1957)
found that aragonite f0'rmation was enhanced under conditions which normally
w0'uld favour calcite if strontium, barium or lead were present. Also, the excess
of carbonate ion prom0'ted increasing am0'unts of aragonite precipitation. Curl
(1962) synthesi,sed these factors by proposing that strontium, magnesium and
other metallic ions may induce the precipitation of aragonite or calcite either by
1) providing isomorphous nuclei, 2) modifying relative growth rates, or 3) by
causing lattice distortions after ads0'rpti0'n, which will lead to the creation of the
crystal lattice dislocations t0' aid gr0'wth. Temperature factors would modify the
rates of these mechanisms. Consequently, the problem 0'f the origin of calcite or
aragonite in any precipitation can be c0'nsidered as a competition between nucleation
and gr0'wth for the two polymorphs. Whichever one can accomplish nucleation
sooner and grow more rapidly will d0'minate.
Calcite is the m0'st comm0'n of all the cave minerals, and forms the bulk of
all major cave f0'rmations. Ultimately, the calcite which f0'rms speleothems comes
from the dissolving of the 0'verlying limestone strata. On and near the surface,
decaying vegetable matter produces carbon dioxide c0'ncentrations, which combine
with water in the soil to produce carbonic acid. The acid dissolves minor amounts
of limest0'ne as it sl0'wly descends to deeper depths. The acidified soil water
dissolves calcium carbonate, until it reaches saturati0'n, all the way down to the
cave. Eventually, part 0'f this water will be exp0'sed t0' the cavern ai,r. The
dissipation of the carb0'n dioxide to the cave air causes a direct precipitation of the
calcite.
The effects of geography and climate are very important with respect to the
relative abundance of calcite formation. The rate at which carbon dioxide is
produced in the soil is proportional to the amount of bacteria and other organisms
promoting decay of the vegetable matter. This, of course, is more prevalant in
the warmer climates, particularly closer to the equat0'r. Likewise, Arctic caves
lack spele0'thems entirely. Also, stalactites will tend to form in the parts of the
cave where carbon dioxide gas will have a better chance 0'f escaping. The better
ventilated portions will have the lower carbon dioxide gas accumulations, and
more spele0'thems.
Despite the fact that many of the great karst areas of the world are in
dolomitised limestone, the mineral d0'lomite is extremely rare as a secondary cave
mineral. Its usual occurrence is as a component of "m0'0'nmilk," which consists
of extremely fine mineral grains held in a relatively stable suspension and having
a milky appearance. The m0'st common species present in moon milk are the
carbonates: calcite, dolomite, huntite, hydr0'magnesite, magnesite and nesqueh0'nite. It is soft, white and like a watery-clay in c0'nsistency, occurring on the
walls of many caves. The predominant minerals are calcite and hydromagnesite.
37
These magnesium carbonates are formed as a result of microorganisms breaking
down the cave wall. Macromonas bipunctata, as well as actinomycetes and algae
are the principal organisms. At least six bacteria species are known to have a part
in the reactions.
Metallic elements are easily . derived from the sulphides in the overlying
limestone. Ground water solutions bring iron, lead, zinc and copper bearing
solutions into contact with the carbonate ions of the cave environment. These new
carbonates are generally quite rare in the cave environment. There are several
recorded deposits, though, of the zinc carbonate, smithsO'nite (Foster, 1951;
Bradbury, 1959; Phillips, 1916; Spencer, 1908). Smithsonite is usually thought of
as the weathering product of the zinc sulphide, sphalerite. There is only one known
observation (Dietrich, 1960) of smithsonite altering to hydrO'zincite. A few limited
regions O'f the world have considerable hydrothermal mineralisation in karst
regions. Cerrusite, a lead carbonate, often occurs in these deposits on the surface
of galena crystals. Only PO'orfarm Cave, West Virginia, U.S.A., has appreciable
depO'sits O'f the iron carbonate, siderite. The copper carbonates, malachite and
azurite, cO'mmO'nly occur in the upper oxidised zone of cO'pper deposits, especially
where limestone is present. In southwestern United States numerous cO'pper mine
shafts have broken intO' small solutional limestone caves, whO'se walls were coated
with these hydrothermally deposited minerals.
THE SULPHATE MINERALS
The sulphate minerals are present in O'ften large quantities in many cave
systems throughout the world. The most common representative of this group is
gypsum. This mineral is apparently restricted to the drier cave passages, and
is almost never found where speleothems are forming. Sometimes after gypsum
formatiO'ns are deposited, the passages have again become moist, causing the
gypsum to dissolve and disappear. In many caves, the sodium sulphate, mirabilite,
is commonly associated.
The origin of the sulphate minerals is a speleological problem currently under
intense investigation by many researchers. Sulphate minerals, particularly gypsum,
are present in many limestone caves throughout the world. In fact, many regions
of the wO'rld have cavern development in extensive stratified gypsum karst. HO'wever, when there is. no gypsum in the overlying limestone, the sulphate O'rigin is
theorised from a number of sources. In Mammoth Cave, the world's thill"d largest
cave system, inwashes frOom an arid, Pleistocene, so~l are proposed by Davies and
Chao (1959). Three sulphate minerals: gypsum, barite and more rarely celestite,
are commonly related to these cavern fills. Davies believes that it is probable that
the cavern passages had large pools of water at the time of the fill deposition.
The waters wO'uld be rich in barium, strontium and calcium ions. As the sulphate
ions in the water grew more concentrated, due to continued influx of fills, the
most insO'luble fO'rm, barium SUlphate (barite) would be precipitated. Next would
be the strontium sulphate (celestite), and finally the more soluble calcium sulphate
(gypsum). Consequently, the gypsum speleothems on the walls would be secondary,
having been dissolved from the Ooriginal gypsum outside the cave entrance, and
refO'rmed in the cave. The idea that sulphides elsewhere in the stratigraphic column
(Weller, 1927; Pohl and White, 1965) are responsible fO'r sulphate deposition in
the cave is very popular, and probably offers the best explanation. Disseminated
pyrite is ultimately the majO'r sO'urce, but complex inorganic and organic chemical
factors have been propO'sed to weather and transport the iO'ns. With the weathering
38
of pyrite in a pH condition near neutrality, as it would be if in contact with
carbonates, the sulphate ion should be formed. The bacteria Thiobacillus thioxidans
and T. ferrooxidans are noted (Pohl and White, 1965) in this mechanism to be
significant in lowering the pH of the acidic reaction. The iron would remain in
the sediment as an insoluble iron oxide and hydroxide. Cavern passages are
sparsely distributed generally, and the sulphate bearing solutions would reach
passages on all levels in the limestone, not only the uppermost passages.
In dry air at ordinary temperatures, the magnesium sulphate, epsomite,
tends to lose water, altering to hexahydrite. This process is particularly notable
near the cave entrances. Likewise, mirabilite will rapidly decompose to thenardite.
There are five other known sUlphate minerals occurring in the cave environment. Ardealite, Ca 2 HP0 4 S04 ·H2 0, from Csoklovina Cave, Rumania and
leconite, NaNH4 S0 4 '2H2 0, both represent an early stage in the breakdown of
bat guano (Taylo'f, 1858; Moore, 1970). Pyrite and marcasite, both very common
sulphides, are probably the ultimate source for much of the iron and sulphur
elements found in the various secondary cave minerals. Wilson Cave, Nevada,
U.S.A., has the iron sulphate, melanterite, associated with stalactitic limonite. This
limestone cave, in particular, has thick lenses of massive pyrite in the strata above
and to the side of the void (Young, 1915). Likewise, this origin would account
for the jarosite found in Tintic Cave, Utah (Stringham, 1946). The rare mineral
b,.:udantIte, PbFesAs04 S04 (OH)6' is reported from only one cave. It is probably
an oxidation product of galena, pyrite and arsenopyrite in the overlying strata,
and transported to this Virginian cave by circulating ground waters.
THE SULPHIDE MINERALS
The lead, zinc and iron elements form the only known sulphide mineral
species in the cave environment. In certain instances, the concentration of the
minerals is great enough, and extensive enough, to promote commercial mining
of the cave. The crevice lead-zinc deposits of northwestern Illinois dolomites are
a case in point. These deposits were formed by the following genesis: 1) a period (s)
of diastrophism that fractured the rock, 2) erosion during which ground water
dissolved the carbonate rocks along joints and in brecciated zones, thus development
of solutional caves along joints, enlarging caverns and honeycombed rock, 3) influx
of hot hydrothermal solutions from depth into the zones of low pressure near the
surface, and finally 4) precipitation of the sulphides in the breccia zones, caverns
and open joints. The principal sulphides formed are the lead SUlphide, galena,
and the zinc sulphide, sphalerite (Bradbury, 1959). The iron sulphides, pyrite
and marcasite, as well as calcite are often associated.
There is one reported (Kaye, 1959) occurrence in a cave of another sulphide,
pyrrhotite. Its identification, however, is uncertain, as it occurs only in trace
amounts as small magnetic platy grains in Playa Pajaro Cave on the Isla Mona,
Puerto Rico.
V ANA DATES AND HALIDES
Uranium and vanadium minerals are very rare in the cave environment,
though several species are reported from the Tyuya-muyun Cave in Fergana,
U.S.S.R. (Chirvinsky, 1925). Cavities in the wall were lined with botryoidal,
crystalline masses of calcite, malachite, barite, turanite and other minerals. Masses
39
of dark-brown compact limestone were cut by coarsely crystalline calcite. Associated
with these are lenses of tyuyamunite, Ca (U02)2(V04)2'nH20, which occurs as
secondary scattered platy grains. Sometimes the mineral forms compact cryptocrystalline masses. As pDinted out, turanite, a copper vanadate, and ferghanite,
a uranium vanadate, are believed to' be leachate products of the tyuyamunite.
Vanadinite is another rare cave species known only in two caves: Havasu Canyon
Cave, Arizona, US.A., and a cave encountered in the Kopje No. 1 Mine in Broken
Hill, Rhodesia (McKee, 1930; Spencer, 1908). In Broken Hill, this mineral is
associated with hopeite, hemimorphite and tarbuttite as encrustation on bone
breccias.
Fluorite is a common mineral in the Halide Group, and of widespread
occurrence. It is, however, only known from two localities. These are Kootenay
Florence Cave, British Columbia, Canada, and in Spirit Mountain Cave, Wyoming.
Also in this category is halite, whose occurrence in caves is limited to Australia.
THE PHOSPHATE MINERALS
The phosphate minerals are the largest group of cave minerals, but contain
some of the rarest species. The ultimate SDurce of the phosphate is the decomposition of bat guano. Fresh bat guano contains about 4 per cent phosphorous
GUANO
BRUSHITE
CoHPO~ ·2H 20
WHITLOCKITE
/
AND BONES
PHOSPHATE
CRANDALLlTE
C03(PO~l2
CoAI 3(P04 l 2 (OHl s ·HzO
H,K 3Al s (P04 l. · 18H z O
MONETITE CoHPO~
TARANAKITE
FLUORAPATITE
VARISCITE AIP~ ·2H 20
Cos(P04 l 3 F
Cos(P04 l 3 (OHl
HYDROXYLAPATITE
/
CARBONATE - APATITE
MARTlNIT[
I
/
CALCIUM CARBONATE
I
CO,o(PO~l 6C03 · H 20
C<I3PO~ C0 30H
SCHOLZITE
C03Zn(PO~lz(OHl2 · H20
\1
HIBBENITE
HOPEITE
Zn
PARAHOPEITE
SALMOITE
(P0 4 l4 (OHl z ·7H O
z
I
ALUMINUM
Zn 3(P04l z ·4H zO
BASIC ZINC PHOSPHATE
SPENCERITE
TARBUTTlTE
TINCTICITE
ZINC AND
7
Zn 3(P04 l z · 4H z O
Zn~(PO~lz(oHlz ·3H zO
Zn z P04 0H
Fe3(PO~lz(oHl3 · 3HzO
IRON
SULFIDES
Table 1. Phosphate minerals derived from the reaction of bat guano with wall rock, clay fill,
and sulphide mineralised ground water and wall rock.
40
CLAYS I
and 12 per cent nitrogen. Consequently, a series of nitrate and phosphate minerals
is possible. The known cave minerals derived from leached guano interractions
with the wall rock, with cave fill and with mineralised solutions, are listed in
Table 1. The leached bat guano products typically occur as thin crusts and earthy
textured aggregates below and adjacent to mounds of dessicated guano. Several
of the zinc phosphates are often found as crystals a few millimetres in length, in
cavities in decaying bat bones, and lining cracks and cavities in bone matrix
deposits. The cave phosphate minerals are still a field of active research in
speleology, as most of their stability relationships are still unknown. In fact, it
is quite probable that several new species will be found, in the future, in this
group of minerals.
OXIDES AND HYDROXIDES
Iron and manganese oxides are very common in caves. Goethite is the typical
rusty brown stain found in most caves. Rarely haematite is encountered in the
cavern environment, but is metastable as cold ground water could react with
it to form the mO're stable goethite. The manganese minerals, mainly psilomelane
and birnessite, often form soot-like layers that cO'ver walls and coat cobbles in
cave streams. These deposits are often laminar. Moore and Nicholas (1964) note
the impO'rtant role played by microorganisms in manganese oxide (wad) formation
and deposition. The bacteria utilise the O'rganic part of the complex molecules,
thus freeing the manganese ions and causing the cave water to become supersaturated with manganese near the bacterial colonies. Birnessite or psilomelane
would then be deposited O'n the rock surface at these sites. Sometimes the manganese
oxides will be mixed with oocides of other metals, usually the iron oxides. There
is a reported case of copper and manganese oxides mixing in Calumet Cave,
Arizona (Ransome, 1904). In this southwestern United States cave the damp
walls were coated with black botryoidal growths. Although the black copper oxide
crystalline fO'rm (tenorite) was not observed, its earthy variety, called melacornite,
is probably the minerC;l1 species.
Gibbsite is a low temperature hydrothermal mineral normally found in cavities
in alkalic and aluminous igneous rocks. It often occurs with boehmite and diaspore
in earthy bauxite deposits in paleokarsts. As for specifically occurring in a solutional
cave, it has been tentatively identified in a few Pennsylvanian caves by White and
Ellisher (1959).
The most common oxide cave mineral is ice. Ice is characteristic of glacier
caves, many lava tubes and other "ice caves." It is also found near the entrance
of many solutional limestone caves. The coming of freezing winter temperatures
often fills the entrance passages with countless frozen ice stalactites, stalagmites
and columns. While running water may occasionally be a primary source for ice
formation, ordinary seepage is the usual source. Frost crystals on cave walls are
undoubtedly the result of condensation of the atmospheric mO'i1sture.
THE NITRATE MINERALS
The cave mineral nitrocalcite is the essential component of the so-called
saltpetre caves. Through a relatively simple chemical reaction, this calcium nitrate
is converted into the potassium nitrate usable for gunpowder. Due to the extreme
solubility of potassium nitrate, it is very rarely found naturally in the cave
environment. Its existence has been authenticated only in caves of the arid regions
of Central Australia (Mawson, 1930). Some caves that have saltpetre deposits
show no evidence that they are inhabited by bats. In circumstances like this, the
41
dried urine of cave rats has been postulated as the nitrate source. Two other nitrate
minerals are known from caves. They are nitramite and nitromagnesite.
THE SILICATES AND ALUMINOSILICATES
Secondary quartz is very rare in the cave environment. The presence of
euhedral quartz crystals in Wind Cave, South Dakota (White and Deike, 1962)
is an indicator of a high temperature solution during formation. At low temperatures (cold ground water) the amorphous form of silica is usually found, and
large amounts of opal have been reported in several Black Hills, South Dakota~
caves. Hydrothermal water temperatures of 100 to' 200 C. are necessary for the
growth of quartz crystals, and thus not typical of the cavern environment.
Microcrystalline quartz is quite commonly found in the caves, but merely represents
eroded fragments of chert nodules that were originally in the limestone strata.
A very unusual speleothem exists in Jewel Cave, South Dakota. Deal (1964)
reported that these all-silica helictite-stalactite speleothems represent the precipitation of crystalline quartz on the undersides of chert ledges, and reach lengths of
40mm and diameters of Smm.
0
0
The clay minerals in the cave environment are usually deposited as fill washed
in from the surface. Consequently, they are not truly representative of secondary
mineralisation. This, however, is not always the case, as in many caves, the
source of the clays is uncertain, and can sometimes be shown that it is derived
from the cavern wall rock. Of these, montmorillonite and illite form the bulk of
the cave clay minerals.
Endellite, a species of the kaolinite group, has been reported by Davies and
Moore (1957) to be of secondary origin in Carlsbad Caverns, New MexicO'. The
dehydration product of halloysite, known as metahalloysite, is reported from
Cave Mountain Cave, West Virginia (Haas, 1962). A minor clay mineral group.
the palygorskite group, is also known from caves. Two of its members, attapulgite
and sepiolite, as well as the palygorskite species are recorded in the literature.
The attapulgite occurs in well cemented cave fill in LO'wer Cave of Carlsbad
Caverns, D.S.A.; the sepiolite is known from Zbrazov Cave, CzechoslO'vakia,
associated with huntite; the palygorskite in BrDken Hill Cave, New Zealand. The
zinc silicate, hemimorphite, is known frDm one cave, that cut into by the Kopje
Mine NO'. 1 on Broken Hill, Rhodesia. The hemimO'rphite is present with hDpeite.
tarbuttite and smithsonite as crystallised crusts on the bones and bone-breccia
of the cave floor.
MINERALOGY OF STALACTITES
Of the more than SO mineral species discussed above, almost one-third are
known to have stalactite-stalagmite forms. In several instances these 24 minerals
may fDrm Dnly a small fractiDn of the stalactite. They are:
Hydrozincite
Ice
Malachite
M elanterite
Mirabilite and an unnamed sodium hemicalcium sulphate dihydrate in phase
with the mirabilite
Parahopeite
Scholzite
Smithsonite
Spencerite
Quartz
Aragonite
Azurite
Barite
Beudantite
Calcite
Carbonate-Apatite, var. francolite
Cerrussite
Epsomite
Goethite/limonite
Gypsum
Halite
Hemimorphite
Hopeite
42
CONCLUSION
Speleogenesis is an old, well established study among researchers. The
theoretical aspects of cave mineralogy are just beginning as a discipline. No longer
are researchers merely limiting their efforts to the classic calcite-aragonite genesis
problem. The study of the complex carbonate-organic residue-metal bearing ground
water interactions is becoming an active and challenging field. Technical advances
in instrumentation such as the scanning electron microscope are being applied with
success. Yet, the basic cave mineralogy is still a relatively unknown area. We
have not yet fully detailed what mineral associations are present in the cave
environment reactions, nor do we fully understand elemental sources for many
components. The thermodynamic kinetics of many cave environment mineral
reactions, particularly the phosphates, are still unknown. This, and detailed clay
mineralogical studies, are promising fields of research for future speleologists
and cave-mineral specialists.
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43